Photomasks are used in a wide variety of applications including the fabrication of semiconductor integrated circuits such as ICs, LSIs and VLSIs. Basically, the photomask is prepared from a photomask blank having a chromium based light-shielding film on a transparent substrate, by forming a predetermined pattern in the light-shielding film by photolithography using UV or electron beams. The current demand for a higher level of integration in the semiconductor integrated circuit market has created a need for a smaller pattern rule. The traditional solution is by reducing the wavelength of exposure light.
However, reducing the wavelength of exposure light improves resolution at the sacrifice of focal depth. This lowers the process stability and adversely affects the manufacture yield of products. One effective pattern transfer method for solving the problem is a phase shift method. A phase shift mask is used as a mask for transferring a micro-pattern.
Referring to FIGS. 16A and 16B, a phase shift mask, specifically a halftone phase shift mask is illustrated as comprising a substrate 1 and a phase shifter film 2′ deposited thereon. The mask consists of a phase shifter 2a that forms a pattern on the substrate and an uncovered area 1a of the substrate 1 that is exposed where the phase shifter 2a is absent. A phase difference of about 180° is set between light transmitted by the uncovered substrate area 1a and light transmitted by the phase shifter 2a. Due to light interference at the pattern boundary, the light intensity at the interfering boundary becomes zero, improving the contrast of a transferred image. The phase shift method permits to increase the focal depth for acquiring the desired resolution. This achieves improvements in resolution and exposure process margin, as compared with conventional masks having ordinary light-shielding patterns in the form of chromium film.
Depending on the light transmission of phase shifter, the phase shift masks are generally divided for practical application into full transmission type phase shift masks and halftone type phase shift masks. The full transmission type phase shift masks are transparent to the exposure light wavelength because the light transmittance of the phase shifter section is equal to the light transmittance of uncovered substrate areas. In the halftone type phase shift masks, the light transmittance of the phase shifter section is several percents to several tens of percents of the light transmittance of uncovered substrate areas.
FIGS. 17 and 18 illustrate the basic structure of a halftone type phase shift mask blank and a halftone type phase shift mask, respectively. The halftone type phase shift mask blank shown in FIG. 17 has a halftone phase shift film 2′ formed over substantially the entire surface of a substrate 1. Patterning the phase shift film 2′ results in the halftone type phase shift mask which is shown in FIG. 18 as comprising phase shifter sections 2a forming the pattern on the substrate 1 and uncovered areas 1a of the substrate where the phase shifter is absent. Light that passes the phase shifter section 2a is phase shifted relative to light that passes the uncovered substrate area 1a. The transmittance of the phase shifter section 2a is set to a light intensity that is insensitive to the resist on a wafer or article subject to pattern transfer. Accordingly, the phase shifter section 2a has a light-shielding function of substantially shielding exposure light.
The halftone type phase shift masks include single-layer halftone type phase shift masks featuring a simple structure and ease of manufacture. Some single-layer halftone type phase shift masks known in the art have a phase shifter of MoSi base materials such as MoSiO and MoSiON as described in JP-A 7-140635.
Although the halftone type phase shift mask is an effective means for accomplishing a high resolution in a simple manner, it suffers from the following problem due to the difference between the wavelength at which light exposure through the mask is carried out (exposure wavelength) and the wavelength of light used for defect inspection of the mask itself (inspection wavelength).
Commonly used in the halftone type phase shift masks is a metal and silicon oxynitride film which has the propensity of increasing its transmittance as the wavelength of irradiating light becomes longer. In comparison between the exposure wavelength and the inspection wavelength, the inspection wavelength is longer as the general rule. In the case of a mask adapted for use with an ArF excimer laser with an exposure wavelength of 193 nm, the defect inspection system generally uses a wavelength of around 260 nm, especially about 266 nm, which is longer than the exposure wavelength of 193 nm. The defect inspection system fails in inspection unless there is a contrast between the uncovered substrate area and the phase shifter. If the transmittance of the phase shifter at the inspection wavelength exceeds 50%, its difference from the transmittance of the uncovered substrate area becomes too small to provide a sufficient contrast for inspection, interfering with defect inspection with a satisfactory precision. The above-mentioned problem arising from the difference between the inspection wavelength and the exposure wavelength is pertinent not only to transmittance, but also to reflectance.
As discussed above, the halftone type phase shift mask is desired to have a minimized dependency of transmittance and reflectance on wavelength. To solve the problem, a study has been made on a phase shift film of multilayer structure in which a transparent film (typically oxide film) having a phase shift function is combined with a metal film having a light absorption function. However, the prior art phase shift film of multilayer structure has a substantial difference in dry etching behavior between the transparent film having a phase shift function and the metal film having a light absorption function. This necessitates a combination of etching processes using different etching gases in the mask manufacture, adding to the cost of mask manufacture (resulting from the increased expense of facility, an increased number of steps, and a reduced production yield).
Also, the halftone phase shift film of multilayer structure has the problem that it is impossible to reduce the dependency of transmittance and reflectance on wavelength at the same time. In the structure in which a transparent film having a phase shift function is combined with a metal film having a light absorption function, a defect repairing system having a laser built therein operates so that the energy of laser light concentrates only in the film having a light absorption function, failing to provide satisfactory repair.